1. Field of the Invention
This invention pertains in general to the inspection of boiling water reactor internals and more particularly to the inspection of the underside of a top guide for a boiling water reactor.
2. Related Art
FIG. 1 is a sectional view, with parts cut away, of a typical reactor pressure vessel 10 for a boiling water nuclear reactor. During operation of the boiling water reactor, coolant water circulating in the reactor pressure vessel 10 is heated by nuclear fission produced in the core 12. Feedwater is admitted into the reactor pressure vessel 10 by a feedwater inlet 14 and feedwater sparger 16. The sparger 16 is a ring-shaped pipe that includes apertures for circumferentially distributing the feedwater inside the reactor pressure vessel 10. The feedwater from the feedwater sparger 16 flows downwardly through downcomer annulus 18, which is an annular region between the reactor pressure vessel 10 and the core shroud 20.
The core shroud 20 is a stainless steel cylinder that surrounds the core 12. Core 12 includes a multiplicity of fuel bundle assemblies 22; two arrays of which are shown in FIG. 1. Each array of fuel bundle assemblies 22 is supported at its top by top guide 24 and at the bottom by core plate 26. Top guide 24 provides lateral support for the top of the fuel bundle assemblies 22 and maintains correct fuel channel spacing to permit control rod insertion.
The coolant water flows downward through downcomer annulus 18 and into core lower plenum 28. The coolant water in the core lower plenum 28 in turn flows upward through the core 12. The coolant water enters fuel bundle assemblies 22 wherein a boiling boundary layer is established. A mixture of water and steam exits core 12 and enters core upper plenum 30 under shroud head 32. Core upper plenum 30 provides a standoff between the steam-water mixture exiting the core 12 and entering standpipes 34. Standpipes 34 are disposed atop shroud head 32 and in fluid communication with core upper plenum 30.
The steam-water mixture flows through standpipes 34 and enters steam separators 36, which may be, for example, of the axial-flow centrifugal type. Steam separators 36 substantially separate the steam-water mixture into liquid water and steam. The separated liquid water mixes with feedwater in mixing plenum 38. This mixture then returns to the core 12 via downcomer annulus 18. The separated steam passes through steam dryers 40 and enters the steam dome 42. The dried steam is withdrawn from the reactor pressure vessel 10 via steam outlet 44 for use in turbines and other equipment (not shown).
The boiling water reactor also includes a coolant recirculation system that provides the forced convection flow through the core 12 necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 18 via recirculation water outlet 46 and forced by a centrifugal recirculation pump (not shown) into a plurality of jet pump assemblies 48 (only one of which is shown) via recirculation water inlets 50. The jet pump assemblies 48 are circumferentially distributed around the core shroud 110 and provide the required reactor core flow.
The United States Nuclear Regulatory Commission requires that for nuclear plant license extensions the reactor internals components subject to age degradation be inspected for deterioration through mechanism such as intergranular stress corrosion cracking. The previous method of inspecting the bottom side of the top guide was conducted with a single camera secured back on itself via a piece of tape such that the camera was pointed directly up. Inaccuracies in the motion and inspection angle were common using this method. Visual inspection criteria set forth in the EPRI Boiling Water Reactor Vessel and Internals Project (BWRVIP) Report-03 (Revision 12), requires the camera angle to be placed 30° or less from the perpendicular with a known distance from the inspection piece. BWRVIP-26A and BWRVIP-183 are both applicable to Top Guide examinations. Employing the previous method it was difficult to verify that the inspection was within the inspection criteria.
Accordingly, a new method is desired that can verify that the inspection criteria has been followed. More particularly, a new apparatus is desired that can carry out such a method and maintain a known camera angle and distance from the inspection piece as well as provide rigidity to keep flow induced impact on the inspection process at a minimum if not entirely eliminated.